A case of mucinous carcinoma of the breast with neuroendocrine differentiat
ion in an 89-year-old woman is presented. The patient presented with a rapi
dly growing right breast mass, which she had had for 2-3 years. The tumor,
15 x 8 x 5 cm, was located mainly in the upper outer quadrant. Light micros
copy revealed a pure mucinous carcinoma of type B. Neuroendocrine different
iation was demonstrated by Grimelius stain and chromogranin A, as well as t
he presence of neurosecretory granules. The breast cancer cells were of lum
inal origin and had dedifferentiated to attain neuroendocrine properties.